[mypy] Improve type annotation for visit_insert & visit_delete

- `visit_insert` method should only accept rql.stmts.Insert type nodes
- `visit_delete` should only accept rql.stmts.Delete type nodes
6 jobs for !63 with topic/default/etyperesolver in 1 minute and 45 seconds (queued for 9 seconds)
latest detached
Status Name Job ID Coverage
  Lint
passed black #462519

00:00:27

passed check-manifest #462521

00:00:43

passed flake8 #462518

00:00:45

passed mypy #462520

00:00:40

 
  Tests
passed py3 #462522

00:00:47

passed py3-from-forge #462523

00:00:51